1. Field of the Invention
The present invention relates to a power amplifier, and more particularly, to a power amplifier of a low-power consumption system that has linearity at peak output while increasing efficiency in a most frequently used range, and thus enabling a battery to last longer.
2. Discussion of Related Art
In general, mobile communication terminals, cellular phones, and non-constant envelope systems (for example, code division multiple access (CDMA), wideband-CDMA (W-CDMA), etc.), in which linearity is an important requirement, usually employ class A or class AB power amplifiers.
Considering current technology development trends, future portable terminals would be required to provide not only a conventional voice-oriented service but also various additional services such as voice, Internet, moving picture, electronic signature/measurement/control, and so forth. In order to do so, it is necessary that they are equipped with a complex function and a high transfer rate; however, as such demand for complex function and high transfer rate increases, power consumption has emerged as the most important issue.
Therefore, producing low-power consumption components is necessary for continuously maintaining the international competitiveness of portable terminals and providing various additional services to keep up with the market.
Meanwhile, in a field of technology in which full-scale development is currently underway, core technology on the rise includes chip miniaturization, analog device gain improvement, leakage current prevention, multi-operating voltage and clock gating circuit design, mixed-mode communication, intelligent transmission protocol, and so forth.
Particularly, a power amplifier, which is an indispensable component for transmitting a signal, consumes the most power in a portable mobile communication terminal. Therefore, if a power amplifier circuit used in a mobile communication terminal and a cellular phone is manufactured in such a way that it consumes less power, the result is a low-power consumption system with multiple and complex functions. Such a system can vitalize the mobile communication service industry and provides individuals with many daily activity functions through just one terminal. Furthermore, since multimedia communication using a terminal can proceed for a longer time, restrictions of space and time can be better overcome and the free flow of information throughout society can be enhanced.
FIG. 1 is a schematic block diagram illustrating a conventional power amplifier.
Referring to FIG. 1, the conventional power amplifier comprises a high-power amplification device 1 amplifying an input radio frequency (RF) signal; a mode switch 2 determining consumption currents I1 and I2 of the high-power amplification device 1; and an input impedance matching circuit 3 and an output impedance matching circuit 4 for matching the input and output impedances of the high-power amplification device 1.
Here, a heterojunction bipolar transistor (HBT) array is mainly used for the high-power amplification device 1, which can be a bipolar junction transistor (BJT) array, field effect transistor (FET) array, and so forth.
In general, when an RF output power is less than a direct current (DC) consumption power, a power amplifier shows very low power efficiency. In order to solve this problem, the conventional power amplifier operates in a high-power mode and a low-power mode in which less power is consumed.
In the conventional power amplifier, the consumption currents I1 and I2 in the high-power mode and low-power mode are determined by the mode switch 2, and a supply voltage is determined to be a battery supply voltage (in cellular phones, about 3.4V to 4.2V). Therefore, power consumption is determined by supplied DC current.
When an RF input to the high-power amplification device 1 increases, an RF power output from the device and a consumed DC current both increase. Here, in a high-power mode of about 1W, a voltage swing range for generating the peak output power becomes the battery supply voltage, and a load impedance of about 2 to 5 Ohms is used according to a current swing range.
On the other hand, since the low-power mode has a low output power of about 16 dBm, a high load impedance is used without considerably increasing current consumption, so that an RF voltage swing range becomes similar to a battery supply voltage range. Therefore, power efficiency can be improved.
In addition, in the low-power mode of about 16 dBm, a load impedance at the maximum efficiency is more than about 15 Ohms. Therefore, when the same output impedance is shared between the high-power mode and low-power mode, a consumption current at an operating point is changed by mode switching in both modes, but the peak output power should be satisfied first. Therefore, a load impedance is designed to be appropriate for the high-power mode, so that efficiency in the low-power mode in which an RF input is less than about 16 dBm is hardly more than about 10%.
In addition, when the consumption currents I1 and I2 of the high-power amplification device 1 are further reduced little-by-little, efficiency is improved; however, since plenty of non-linear elements are generated during high-power signal operation, the linearity of the power amplifier is deteriorated.
Consequently, there is a limit to embodying a high-efficiency power amplifier in a low-power consumption system only by changing the consumption currents I1 and I2 while maintaining a high degree of linearity in the high-power and low-power modes.
FIG. 2 is a graph showing Conexant company's use rate function according to output power. When an output power use rate is as shown in FIG. 2, such as in a mobile communication terminal/cellular phone, a power amplifier operating with maximum efficiency at the peak output power has a problem in that a power-added efficiency drops in an area between about −16 dBm and 16 dBm that has a high use rate.